1. Field of the Invention
The present invention relates to a computer system, method, and computer program for aggregation of tree-structured data, and more particularly to a computer system, method, and computer program for creating an index for aggregation of tree-structured data.
2. Description of Related Art
In various search and database fields, there are operations performed for counting values by using an aggregate function in addition to “GROUP BY” and “HAVING” in a relational database (RDB). To perform the counting efficiently, a variety of indices and data formats have been suggested.
Conventional techniques have been successful in general-purpose search and aggregation. In these techniques, however, redundant data is maintained for aggregation. Therefore, the techniques have problems in performance for massive data quantities. Moreover, regarding aggregation using a B-tree (B+-tree) typical in RDB, the conventional techniques have a problem of slow processing in the case of a loose search condition such as, for example, a case where 50 percent of the data is a target of the counting.
Patent Document 1, U.S. Pat. No. 7,287,033, discloses an index structure that is a combination of two types of existing tree-structure indices such as “Layered Index” and “Patricia Tree.” The index structure, however, is unsuitable for the case where there is a need for processing a large number of nodes, although it is effective in narrowing down the number of nodes for a search. Furthermore, a technique disclosed in Patent Document 2, U.S. Pat. No. 7,330,848, uses the data format of a conventional RDB. In view of the above description, it is apparent that there is a need for a technique for counting values efficiently in various search and database fields.